1. Chapter 18 Notes
Volcanism

2. Volcanoes
The location of volcanoes on Earth is not random

3. Volcanoes at Convergent Boundaries
Most volcanoes on land are at subduction zones, characterized by explosive eruptions
Circum-Pacific belt (Ring of Fire; ex: Pinatubo, St. Helens)
Mediterranean Belt (ex: Vesuvius, Etna)

4. Volcanoes at Divergent Boundaries
About 2/3 of the Earth’s volcanic activity occurs underwater at divergent boundaries
Mid-ocean ridges
Non-explosive, produces large amounts of lava

5. Hot Spot Volcanoes
Some volcanoes form over stationary magma plumes (hot spots), not at plate boundaries
As a plate moves over the hot spot, a chain of islands is formed

6. Flood Basalts
Hot spots beneath continental crust can form flood basalts

7. Anatomy of a Volcano
Magma travels from the magma chamber, through a conduit and lava emerges through a vent
Craters form around the vent

8. Volcano Comparison
Appearance of a volcano depends on the type of eruptions and the type of material forming the volcano
Shield: non-explosive
Cinder: small, steep
Composite: explosive

9. Eruptions Characteristics of an eruption are determined by:
Temperature
Pressure
Magma composition (silica content)
Dissolved gases (think about opening a can of pop that has been shaken)
Viscosity (determined by temperature and composition)

10. Types of Magma: Basaltic
Silica content: low (less than 50%)
Viscosity: low
Eruptions: quiet, very frequent
Example: Kilauea, Hawaii

11. Types of Magma: Andesitic
Silica content: 50 to 60%
Viscosity: intermediate
Eruptions: intermediate explosivity
Examples: Tambora, Indonesia; Colima, Mexico

12. Types of Magma: Rhyolitic
Silica content: high (more than 60%)
Viscosity: high, lots of dissolved gas
Eruptions: explosive, very infrequent
Example: Yellowstone

13. Explosive Eruptions
Tephra: solid fragments of rock ejected from a volcanic vent
Ash (less than 2mm)
Bombs
Blocks
Pyroclastic flows: clouds of tephra mixed with hot gases

14. Pyroclastic flow

15. Intrusive Activity
Plutons form from volcanic activity below the Earth’s surface
Batholiths: large, irregular shape
Stocks: small irregular shape
Laccoliths: round top, flat bottom
Sills: parallel to existing rock
Dikes: cut across existing rock

16. Plutons